1. Field of the Invention
The present invention relates to an image capture apparatus which performs control of a lens drive system during image capture, and a zoom lens control method for such image capture apparatus.
2. Description of Related Art
Open-loop control methods using stepping motors as actuators have been widely used in recent camera drive systems. In such a control mechanism, positioning resolution has been increased to such an extent that position accuracy of the order of several μm can be realized.
However, in open-loop drive control using a stepping motor, since it is difficult to know whether a lens which is a target object to be controlled is actually moving according to a required value, an out-of-step phenomenon in which control by a control section is difficult always becomes a large problem. In the open-loop drive control using the stepping motor, even if such out-of-step phenomenon occurs, the stepping motor is regarded as moving accurately, so there is a possibility that a discrepancy occurs between the position of the lens which is a target object to be controlled and a directed position (target value). Once the deviation occurs, the stepping motor becomes unable to perform correct operation so long as special correction processing is not performed.
Accordingly, because the out-of-step phenomenon easily occurs in the open-loop control using the stepping motor, an out-of-step correction function is very important, and it is desirable that “out-of-step” can be detected and corrected at as many positions as possible in an area which is be normally used, i.e., between a wide-angle end and a telephoto end.
Existing determination methods for lens positions are as follows.
A first method is of the type which acquires an absolute position by using a Hall sensor, a potentiometer and the like.
A second method is of the type which uses a reset sensor for detecting a reference position and an FG (frequency generator) pulse counter for counting pulses from the reference position, the reset sensor and an MR (magnetoresistive) sensor, or the like, and acquires a relative position with respect to the reference position by a combination of the reset sensor and an encoder.
A third method is of the type which detects a plurality of reference positions by using a plurality of reset sensors such as two reset sensors or three reset sensors.
If the first method is used, feedback control can be introduced, so that the issue of “out-of-step” can be avoided. However, the first method has the issue that noise components easily enter a sensor itself, a cost issue, and the size issue that the first method is disadvantageous for reductions in size of lens systems. A decrease in positioning accuracy due to noise is particularly a fatal issue.
In the second method as well, once a reference position is determined by a reset sensor, feedback control using an encoder can be performed to avoid the issue of “out-of-step”. However, there are issues such as increased costs and the issue that the second method is disadvantageous for reductions in size of lens systems.
In the third method, it is possible to realize at low cost and with comparatively high position accuracy a determination as to whether the output of a reset sensor is at a high level or a low level, which determination is based on whether the reset sensor is interrupted by a interrupting section. Stepping motors which are currently widely used as actuators for driving zoom lenses of cameras do not need absolute position detection, because of their open-loop control. Accordingly, in many cases where stepping motors are used, a method of the type which uses a plurality of reset sensors as in the third method is adopted.
A 2-reset-sensor type using two reset sensors is currently general in a combination of the third method and a retractable lens system in which a movable zoom lens tube is movable between a retracted state and a projected state with respect to a lens barrel. The term “2-reset-sensor” means, as indicated by the name, a structure of the type having two reset sensors for position detection. In general, the 2-reset-sensor type uses two reset sensors each having one switching point (edge) between its output levels, and can perform positioning necessary for open-loop control of a retractable lens by means of a combination of such two reset sensors or by detecting a switching point (edge) between the output levels of each of the two reset sensors.
The functions necessary for the retractable lens system are first to fourth ones which will be listed below.
The first function is to satisfy on a mechanism basis that the movable zoom lens tube is in the retracted state with respect to the lens barrel. The second function is to initialize the position of the movable zoom lens tube near a wide-angle end without out-of-step when the movable zoom lens tube is moved forward with respect to the lens barrel. The third function is to satisfy on a mechanism basis that even if a focus lens is driven to move in the lens barrel through its entire operation range, a zoom lens is located in an noninterference area where the zoom lens and the focus lens do not collide with each other. The fourth function is to enable a control section to perform position control during out-of-step between the wide-angle end and a telephoto end when the movable zoom lens tube is moved with respect to the lens barrel from a retracted state to a projected state.
FIGS. 8A and 8B are diagrams showing the output of a conventional basic 2-reset-sensor structure 1. FIG. 8A shows the output of a reset sensor 1, while FIG. 8B shows the output of a reset sensor 2.
A method of realizing the first to fourth functions necessary for the retractable lens system by basic combinations of reset sensor output levels shown in FIGS. 8A and 8B will be described below. A retraction end determination shown at 81 is carried out on the basis of whether the output of the reset sensor 1 shown in FIG. 8A switches from its high level to its low level or from the low level to the high level. An area including the retraction end determination 81 and taking a mechanical backlash into account forms a retraction area 85.
Zoom-position initialization shown at 82 is carried out by detecting in which area a zoom lens is currently located between a wide-angle end 86 and a telephoto end 87, on the basis of a combination of the low level and the high level or a combination of the low level and the low level, which are the outputs of each of the reset sensor 1 shown in FIG. 8A and the reset sensor 2 shown in FIG. 8B. According to this process, before initialization of a zoom position is performed, it is possible to determine whether the focus lens and the zoom lens can be driven, thereby reducing the time required to initialize the focus lens and the zoom lens and greatly contributing to a total reduction in the start time of a camera.
A determination as to a zoom lens/focus lens noninterference area shown at 83 and out-of-step position correction shown at 84 are performed between a wide-angle end 86 and a telephoto end 87 on the basis of a determination as to switching between the output levels of the reset sensor 2 shown in FIG. 8B.
FIGS. 9A and 9B are diagrams showing the output of a conventional basic 2-reset-sensor structure 2. FIG. 9A shows the output of a reset sensor 1, while FIG. 9B shows the output of a reset sensor 2.
In the method shown in FIGS. 8A and 8B, in the case of a lens system in which a zoom lens/focus lens noninterference area 93 shown in FIGS. 9A and 9B is not in the vicinity of a wide-angle end 96 but out-of-step position correction 94 is between the wide-angle end 96 and a retraction area 95, because the combination of the reset sensor outputs becomes as shown in FIGS. 9A and 9B, there occurs a restriction in which the out-of-step position correction 94 and zoom-position initialization 92 are not performed so long as the zoom lens is not driven from the wide-angle end 96 toward the retraction area 95.
In the case of a structure in which one reset sensor has one edge, a position where out-of-step position correction can be effected is substantially limited to one position. Accordingly, even if there is a position where the zoom lens easily becomes out of step near the telephoto end 97, it is difficult to effect out-of-step position correction at the position.
A stepping motor control method is also disclosed (refer to Patent Document 1 (Japanese Patent Application Publication JP H11-110045). In this method, feedback control of a stepping motor is performed, and a zoom control section refers to a zoom lens operation switch, a reference position detection signal of a zoom reset sensor, and a current position detection signal of a zoom lens outputted from a detection circuit, and outputs a control signal for the zoom lens to a motor driver to drive a zoom motor. A zoom control section determines whether the zoom motor is operated according to the control command.